OslcXmlProvider / OslcRdfXmlProvider always returns true for method "isReadable"

[jhemm2]

In the OslcRdfXmlProvider hence also in the OslcXmlProvider the isReadable method always returns true.
This makes it impossible to use another XML Messagebody reader.

isReadable method should be implemented correctly, e.g.:

`    @Override
    public boolean isReadable(final Class<?> type, final Type genericType, final Annotation[] annotations,
            final MediaType mediaType) {
        return isReadable(type, mediaType, OslcMediaType.APPLICATION_RDF_XML_TYPE, OslcMediaType.APPLICATION_XML_TYPE,
                OslcMediaType.TEXT_XML_TYPE, OslcMediaType.TEXT_TURTLE_TYPE);
    }`

[berezovskyi]

Thank you for your report, @jhemm2, and a suggested fix. Would you be able to make a pull request, please? Also, do we need to do the same fix in other classes?

[berezovskyi]

@jhemm2 do you consider this fixed by #286 and #288?